Before buying memory, we have to look at its price, but also at its frequency, timing, and particles. The most basic thing is to see what generations of DDR the memory is. When it comes to DIMM, I believe not many people have talked about it, let alone seen businesses add the specification to the product details. So, what is DIMM?

Ⅰ Beginning of Modular: SIMM

Before introducing DIMM, we need to know what is SIMM, which is called Single In-line Memory Module, and as the name suggests, the structure of this memory is designed to be modular.

At the beginning of the 1980s (to the end of the 1990s), SIMM has been used in computers. Compared to today's capacity of a few gigabytes, a few megabytes were big memory back then.

32-pin SIMM memory (8bit) generally has 256KB ~ 4MB. Since the CPU is 16bit, it must also be used in pairs, and in the case of a 32bit processor, 4 sticks of memory need to be inserted!

72 pin SIMM

And 72-pin memory (32bit) is generally 4 ~ 64MB, which can be used on 386DX, 486DX, and Pentium 586. There is also some 64-pin dedicated SIMM memory from GVP and Apple.

Ⅱ Coexistence of SIMM and DIP

Although the structure of SIMM has been advanced, the early 8088, XT, and AT computers, did not use this type of memory. Instead, the above DIP "dual in-line package" memory was used, which looked like a centipede.

In the 80286 era, along with the rising demand for hardware and software, DIP memory capacity was clearly not enough, and SIMM was gradually accepted.

But SIMM also has its problems, such as its two sides of the gold finger are actually interoperable, the utilization rate is not high. The DIP memory also coexisted with the modular memory sticks for a long time.

Ⅲ DIMM appeared

DIMM is known as a Dual in-line memory module. From the name, we know that the "single" is changed into "double", so what difference can there be?

In fact, the DIMM volume has become larger and the length has become longer. Upgrade from 32bit to 64bit, 168 pins, each side 64Pin and independent, independent of the transmission signal, the voltage from 5V down to 3.3V, providing a large capacity of 32MB ~ 1GB. Since then the memory has entered the DIMM era? No, it is the SDR SDRAM era.

What is SDR SDRAM? The full name is Single DataRate SDRAM, and simply put, it features "synchronization".

DIMM

SDRAM has a synchronous interface with a pipeline mechanism, which is equivalent to:

Originally, each traffic light intersection can only stop one car, the car drove through to let the next car in. Now, the intersection is lengthened. There are more cars waiting for the traffic light. Once the light turns green, all the cars can drive away together.

And this waiting time for the light is called Latency, which we often see when running AIDA64 memory tests in ultra memory. (Previously, the memory latency was fixed)

Ⅳ DIMM types

Since the advent of laptops, memory has also had to take into account size and power consumption. Using the full-size memory of a regular PC didn't work well, so SO-DIMM was developed. Laptop memory became even smaller in terms of the number of pins.

Normal memory uses a short wire connection topology (Stub-bus). The data exchange between it and the Northbridge chip controller is through the 64bit parallel bus, so it may be interfered with by adjacent lines. Therefore, FB-DIMM (Fully Buffered DIMM) appeared. FB-DIMM adds a control chip for data transfer and read-write control, which increases stability, speed, and capacity density. It is mainly used for servers.

In the Pentium 4 era (478, Intel i850), Intel also jointly launched Rambus DRAM (RDRAM), which added RISC streamlined instruction set, the purpose is high frequency!

RDRAM

You know that DDR has not yet come out at that time, its frequency has reached 1066MHz. The mainstream DDR is only 400MHz. But history is always the same. In the end, RambusDRAM was defeated by AMDK7+DDR because of its high price and consumers did not buy it.

UDIMM

Full name: Unbuffered DIMM

Features: no buffers or registers → faster latency; only low-density modules (due to error issues); support 2DPC, each DIMM supports up to 2 levels

Main application: desktop computer

SODIMM

Full name: Small Outline DIMM

Features: smaller than ordinary DIMM; for systems with limited space

Main application: notebook

DIMM used by the server

First of all, we should understand that server memory is also memory (RAM), with some unique technologies that have extremely high stability and error correction performance. The main technique is to introduce some new unique technologies on the memory, such as Parity, ECC, Register technology, etc...

Main techniques used by server DIMM:

(1) Parity (parity check): In ordinary memory, a technique is often used. Parity check codes are widely used in error detection codes. They add a check bit to each character (or byte) of the data, and can detect a character All of the odd (even) errors in the same position in the computer, but Parity has a shortcoming. When the computer finds an error in a Byte, it cannot determine which position the error is in and cannot correct the error.

(2) ECC (Error Correcting Code): ECC itself is not actually a type of memory, nor is it a dedicated memory technology. It is a computer instruction widely used in various fields, and it is an instruction error correction technology. From this name, we can see it Its main function is to "find and correct errors". It is more advanced than parity correction technology mainly in that it can not only find errors, but also correct these errors. After these errors are corrected, the computer can perform the following tasks correctly to ensure that the server is normal run. The reason why it is not a memory model is because it is not a technology that affects the memory structure and storage speed. It can be applied to different memory types.

(3) Register (REG): We can understand the function of Register on the memory as a book directory. With it, when the memory receives a read and write command, it will retrieve this directory first, and then perform read and write operations, which will greatly Improve server efficiency. The memory with Register must have Buffer, and the Register memory that can be seen also has ECC function, which is mainly used in high-end servers and graphics workstations, such as IBM Netfinity 5000.

RDIMM

Full name: Registered DIMM (two-wire memory module with register)

Features: A register is placed between the system memory controller and DRAM to control the address and command signal of the DIMM; an 8-bit parity check signal is added for error correction; supports x8, x4 DRAM / supports 3DPC, each DIMM supports at most 4 levels.

Main application: server

LRDIMM

Full name: Load-Reduced DIMM

Features: The register is replaced with a memory buffer to reduce the load; the delay is better, and it is more suitable for high-density modules

Main application: server

ECC UDIMM / ECC SODIMM

Full name: Error Correction Code UDIMM / SODIMM (UDIMM SODIMM with ECC)

Features: able to detect and correct data corruption and reduce the number of crashes

Main application: server

VLP RDIMM / VLP ECC UDIMM

Full name: Very Low Profile RDIMM / ECC UDIMM (narrow strip)

Features: The height is lower than normal DIMM; it takes up less space and has better heat dissipation, but it is not suitable for high-density modules

Main application: server (blade)

Ⅴ DDR emerged

DDR, whose full name is Dual Date RateSDRAM, is an improved upgrade of SDR. SDRAM is a storage mechanism and DIMM is a module type.

In the beginning, DDR mainstream capacity is 128MB ~ 1GB, mainstream frequency is 400MHz. Later, DDR support dual-channel after the bandwidth doubled. The ending is known to all, DDR finally beat RDRAM, and there is now DDR4.

DDR4 8G

In fact, strictly speaking, the so-called "four generations of memory" should refer to DDR3, while the first generation is SDR SDRAM, and DDR4 is already the fifth generation.

Ⅵ Variants of DDR4

We know that Intel's official paperwork says that this generation of motherboards supports 128GB of memory, but there are no single 32GB of memory available on the market. 4 slots or even 2 slots of small boards are not very realistic to insert full on the big memory unless it is an 8-slot like HEDT.

In 2019, ASUS, Chipset, and Zadak also worked together on a non-JEDEC standard, DC-DIMM (Double Capacity DIMM), which is the so-called "double capacity strip" that doubles the upper limit of memory capacity.

It is essentially two memory to do together, but most motherboards do not support this standard. Supported motherboards are ASUS's own M11E, M11G, but they are very expensive.

Ⅶ When will DDR5 come?

Each generation of DDR update is accompanied by a doubling of the number of memory prefetch bits (2bit-4bit-8bit), which brings a doubling of memory bandwidth. By the DDR4 era is still using the DDR3 8bit, but the bandwidth is still doubled, why? Because DDR4 uses an increase in the number of Bank (memory banks), multiple BankGroup (BG) programs, a group of BG only 8bit, then 4 groups is 32bit.

So how will DDR5 be?

Recently, AMD and Intel have started to support DDR5 when releasing new products in 2021.

In July 2020, the Joint Electron Device Engineering Consortium (JEDEC) finally officially released the final specification for the next generation of mainstream memory (DDR5 SDRAM). Since the late 1990s, double data rate (DDR) memory technology has gone through several iterations and has driven the rapid development of PCs, servers, and other ecologies.

It is reported that JEDEC originally proposed the DDR5 memory specification in 2018, two years after the official version came a little late, but it does not diminish the importance of the new generation of memory standards.

It is worth mentioning that DDR5 again focuses on the density and rate of DRAM storage. The maximum memory rate is set to 6.4Gbps to start with, and the single LRDIMM capacity is expected to reach 2TB.

Compared to DDR4 memory, the DDR5 standard already supports a single 64Gbit DRAM memory chip, which is four times the maximum allowable capacity of DDR4 (16Gbit).

Combined with the core stacking process, up to 8 sets of cores can also be stuffed into a single chip so that 40 units of LRDIMM can reach 2TB of effective storage capacity. As for the less eye-catching unbuffered DIMMs, a typical dual-sided configuration can also achieve 128GB of single memory capacity.

For DDR5 memory, platform support is the biggest issue, but from the perspective of products announced by major companies so far, most new products have officially supported DDR5 memory platforms.